Persistent memory replication in RDMA-capable networks

ABSTRACT

A mechanism is provided in a data processing system comprising at least one processor and at least one memory, the at least one memory comprising instructions that are executed by the at least one processor and configure the at least one processor to implement a replication protocol for replicating memory from an initiator to a target. The initiator requests one or more pages of memory at the target. Responsive to the initiator receiving a page advertisement from the target, the initiator updates a list of pages at the initiator. The list of pages is replicated at the target. The initiator performs a remote direct memory access (RDMA) write to the one or more pages of memory at the target. Responsive to successful completion of the RDMA write, the initiator updates the list of pages at the initiator. Upon completion of the RDMA write the list of pages is updated at the target.

BACKGROUND

The present application relates generally to an improved data processingapparatus and method and more specifically to mechanisms for persistentmemory replication in remote direct memory access (RDMA) capablenetworks.

InfiniBand™ is an industry-standard specification that defines aninput/output architecture used to interconnect servers, communicationsinfrastructure equipment, storage and embedded systems. A true fabricarchitecture, InfiniBand (IB) leverages switched, point-to-pointchannels with data transfers that generally lead the industry, both inchassis backplane applications as well as through external copper andoptical fiber connections. Reliable messaging (send/receive) and memorymanipulation semantics (remote direct memory access (RDMA)) withoutsoftware intervention in the data movement path ensure the lowestlatency and highest application performance. Infiniband APIs andprotocols can be used on Ethernet Fabric, when ROCE transport (RDMA overConverged Ethernet) is deployed.

This low-latency, high-bandwidth interconnect requires only minimalprocessing overhead and is ideal to carry multiple traffic types(clustering, communications, storage, management) over a singleconnection. As a mature and field-proven technology, InfiniBand is usedin thousands of data centers, high-performance compute clusters andembedded applications that scale from two nodes up to clusters utilizingthousands of nodes. Through the availability of long reach InfiniBandand Fast Ethernet over Metro and wide area network (WAN) technologies,InfiniBand and ROCE are able to efficiently move large data between datacenters across the campus to around the globe.

DMA can also be used for “memory to memory” copying or moving of datawithin memory. Either source or destination memory can be IO memory thatbelongs to a hardware device (for example PCI IO memory). DMA canoffload expensive memory operations, such as large copies orscatter-gather operations, from the CPU to a dedicated DMA engine. Animplementation example is the I/O Acceleration Technology. Without DMA,when the CPU is using programmed input/output, it is typically fullyoccupied for the entire duration of the read or write operation, and isthus unavailable to perform other work. With DMA, the DMA master firstinitiates the transfer, and then it does other operations while thetransfer is in progress, and it finally receives notification from theDMA slave when the operation is done. IO accelerators typically havededicated DMA master engines, which allow the hardware to copy datawithout loading the CPU. This feature is useful at any time that the CPUcannot keep up with the rate of data transfer, or when the CPU needs toperform useful work while waiting for a relatively slow I/O datatransfer. Many hardware systems use DMA, including disk drivecontrollers, graphics cards, network cards and sound cards. DMA is alsoused for intra-chip data transfer in multi-core processors. Computersthat have DMA channels can transfer data to and from devices with muchless CPU overhead than computers without DMA channels. Similarly, aprocessing element inside a multi-core processor can transfer data toand from its local memory without occupying its processor time, allowingcomputation and data transfer to proceed in parallel.

Remote direct memory access (RDMA) is a direct memory access from thememory of one computer into that of another without involving eitherone's operating system. This permits high-throughput, low-latencynetworking, which is especially useful in massively parallel computerclusters. RDMA supports zero-copy networking by enabling the networkadapter to transfer data directly to or from application memory,eliminating the need to copy data between application memory and thedata buffers in the operating system. Such transfers require little workto be done by CPUs, or context switches, and transfers continue inparallel with other system operations. When an application performs anRDMA Read or Write request, the application data is delivered directlyto the network, reducing latency and enabling fast message transfer.However, this strategy presents several problems related to the factthat the target node is not notified of the completion of the request(single-sided communications).

RDMA capable applications exchange messages via objects called queuepairs (QPs). Each QP comprises of send and receive queue, and in orderto exchange messages, the local and remote QPs need to connect to eachother. The process of connection establishment involves sending andreceiving connection management (CM) management datagrams (MADs) and iscovered by Infiniband™ Specification.

Applications can use RDMA technology only after they have establishedreliable connections. Modern RDMA adapters are powerful, and it is notpossible to utilize their power without use of multiple hardware eventqueues and multiple application threads. For example, a dual-port 100Gbit adapter can process 6 million sends and 6 million receives persecond (using message sizes of 4 KB). Such adapters have at least 100events queues, and commodity servers with that many CPUs are widelyavailable. One of the scalable approaches to utilize Interconnect andCPU performance is to use multi-domain approach, where each applicationthread opens its own device context and binds to its own device eventqueue. Each thread can pin to a given CPU and pin event queue to receiveinterrupts on the same CPU. This approach minimizes context switches,cross-CPU communication and cross-CPU locks, allowing maximization ofsystem performance. At the same time, it requires each applicationthread to establish connections of its own.

To implement failover and data redundancy, modern data-centerapplications may replicate memory. For example, storage writetransactions can be replicated to a number of backup nodes beforeacknowledgment of the write request is returned to the initiator. Tradetransactions can be mirrored to backup trading servers before beingacknowledged. Databases may replicate journal or other transactionsbefore completing the store operations. All these applications strive toachieve minimal latency while consuming minimal CPU resources. The useof RDMA for these applications allows meeting these requirements.

Applications that use RDMA for memory replication typically deploy oneof the two approaches:

1. Use of conventional storage protocols that supports RDMA. Examples ofsuch protocols include SRP (SCSI RDMA protocol), ISER (ISCSI RDMAExtensions) or XBAND protocol deployed by XIV enterprise storage. Inthese protocols, initiator (a party that wants to replicate), sends arequest to target. Request specifies the source addresses and theirkeys, and the information regarding what is being replicated. When usingSRP or ISCSI—which are standard storage protocols—the destination may bea virtual storage volume in memory (the volume ID and offset within thevolume), that correspond to the source memory that is replicated. Whenusing XBAND, a more direct representation of what is the transactionthat is being replicated is possible. The target then may allocatememory at destination and perform a set of RDMA read operations frominitiator to target. When RDMA read application are complete, a replymessage is sent to the initiator regarding the status of the transfer.This approach suffers from several performance limitations:

Multiple messages are done for one transfer that consume resources onboth initiator and target: initiator send—target receive—target RDMAread—target send reply—initiator receive reply. This is opposed to thesingle RDMA transaction (if it can be done) to a pre-negotiated addressfrom initiator to target.

RDMA reads are more expansive then RDMA writes. Implementation that cando RDMA write for memory replication would be more efficient.

Memory allocations per IO on target can be expansive.

2. Use of active-to-passive memory replication to a static memory log onpassive remote is another approach. In this approach, a standby instanceof the application runs on a remote node. When new passive instance isstarted, the active instance and remote instances connect. Remoteinstance allocates a static memory log and exchanges the size of log andits address with the active instance. More than one instance of memorywindow, as their dynamic addition or resizing is possible. The activeinstance of the application will replicate its transactions to one ormore memory windows provided by the target. Shall active applicationfail, the standby application will assume active role and will restarttransactions from the last known positions in the memory logs. Thisapproach has advantage of good performance (no allocations per IO, RDMAwrites as opposed to RDMA reads, and single initiator operation oninitiator). The disadvantages of this approach are inability to deployactive-to-active implementations and poor error recovery. Upon a singlereplication error to a standby instance, it is assumed that a wholememory log is lost and it needs to be re-synchronized.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described herein in the DetailedDescription. This Summary is not intended to identify key factors oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

In one illustrative embodiment, a method is provided in a dataprocessing system comprising at least one processor and at least onememory, the at least one memory comprising instructions that areexecuted by the at least one processor and configure the at least oneprocessor to implement a replication protocol for replicating memoryfrom an initiator to a target. The method comprises requesting, by theinitiator, one or more pages of memory at the target. The method furthercomprises, responsive to the initiator receiving a page advertisementfrom the target, updating a list of pages at the initiator. The list ofpages is replicated at the target. The method further comprisesperforming, by the initiator, a remote direct memory access (RDMA) writeto the one or more pages of memory at the target. The method furthercomprises, responsive to successful completion of the RDMA write,updating the list of pages at the initiator. Upon completion of the RDMAwrite the list of pages is updated at the target.

In other illustrative embodiments, a computer program product comprisinga computer usable or readable medium having a computer readable programis provided. The computer readable program, when executed on a computingdevice, causes the computing device to perform various ones of, andcombinations of, the operations outlined above with regard to the methodillustrative embodiment.

In yet another illustrative embodiment, a system/apparatus is provided.The system/apparatus may comprise one or more processors and a memorycoupled to the one or more processors. The memory may compriseinstructions which, when executed by the one or more processors, causethe one or more processors to perform various ones of, and combinationsof, the operations outlined above with regard to the method illustrativeembodiment.

These and other features and advantages of the present invention will bedescribed in, or will become apparent to those of ordinary skill in theart in view of, the following detailed description of the exampleembodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, as well as a preferred mode of use and further objectivesand advantages thereof, will best be understood by reference to thefollowing detailed description of illustrative embodiments when read inconjunction with the accompanying drawings, wherein:

FIG. 1 depicts a pictorial representation of an example storage systemin which aspects of the illustrative embodiments may be implemented;

FIG. 2 is a block diagram of an example data module in which aspects ofthe illustrative embodiments may be implemented;

FIG. 3 is a block diagram of just one example data processing system inwhich aspects of the illustrative embodiments may be implemented;

FIG. 4 is a block diagram illustrating the components of an initiator ina protocol for efficient memory replication using RDMA in accordancewith an illustrative embodiment;

FIG. 5 is a block diagram illustrating the components for a target in aprotocol for efficient memory replication using RDMA in accordance withan illustrative embodiment;

FIGS. 6A and 6B are flowcharts illustrating operation of an initiatorperforming an RDMA request in accordance with an illustrativeembodiment;

FIG. 7 is a flowchart illustrating operation of an initiator performinga commit request in accordance with an illustrative embodiment;

FIG. 8 is a flowchart illustrating operation of the initiator performinga page request in accordance with an illustrative embodiment;

FIGS. 9A-9C are flowcharts illustrating the operations of mechanisms forhandling completions in accordance with an illustrative embodiment;

FIG. 10 is a flowchart illustrating operation of a mechanism forhandling target page advertise messages in accordance with anillustrative embodiment;

FIG. 11 is a flowchart illustrating operation of a mechanism for targethandling of a page advertisement in accordance with an illustrativeembodiment;

FIG. 12 is a flowchart illustrating operation of a mechanism for targethandling of a page request in accordance with an illustrativeembodiment;

FIG. 13 is a flowchart illustrating operation of a mechanism for targethandling of a commit message in accordance with an illustrativeembodiment;

FIG. 14 is a flowchart illustrating operation of a mechanism for targethandling of a page advertise completion in accordance with anillustrative embodiment;

FIG. 15 is a flowchart illustrating operation of a mechanism fordisconnect handling in accordance with an illustrative embodiment;

FIG. 16 is a flowchart illustrating operation of a mechanism forhandling target dead node event by the initiator in accordance with anillustrative embodiment;

FIG. 17 is a flowchart illustrating operation of a mechanism forhandling initiator dead node event by target in accordance with anillustrative embodiment;

FIG. 18 is a flowchart illustrating operation of mechanism for initiatorand target resume in accordance with an illustrative embodiment;

FIGS. 19a and 19b are flowcharts illustrating operation of a mechanismfor sending a login message from timer in accordance with anillustrative embodiment;

FIG. 20 is a flowchart illustrating operation of a mechanism forretrying login from the timer in accordance with an illustrativeembodiment;

FIG. 21 is a flowchart illustrating operation of a mechanism forhandling login message response in accordance with an illustrativeembodiment;

FIG. 22 is a flowchart illustrating operation of a mechanism forhandling a login message request in accordance with an illustrativeembodiment;

FIG. 23 is a flowchart illustrating operation of a mechanism forretrying login response from timer in accordance with an illustrativeembodiment; and

FIG. 24 is a flowchart illustrating operation of a mechanism forhandling a login response completion in accordance with an illustrativeembodiment.

DETAILED DESCRIPTION

The illustrative embodiments provide mechanisms for an efficient storageprotocol for replicating memory. The protocol can use remote directmemory access (RDMA) and rely on local completion to signal messagedelivery. The protocol is persistent and will retry until the peer isreported inactive. The protocol supports an infinite number of messagesand shared interconnect queues. Key features of the protocol include anRDMA page pool, reconnect synchronization, and posted, retry, andpending queues.

A standard storage protocol works through the following steps: 1) theinitiator sends a replication request to the target describing thesource memory addresses, 2) the target does an RDMA read from theinitiator, and 3) the target sends an acknowledgement to the initiator.The illustrative embodiments use RDMA writes to pre-agreed-upon memoryof the target and inform the target of performed transactions (informingcan be aggregated). Completion of a transaction is judged by receivinghardware acknowledgements of completed RDMA write or completed sendrequests. The approach of the illustrative embodiments has lower latencyand consumes less processor overhead on the initiator and the target.

To make the solution of the illustrative embodiments workable, a memorynegotiation scheme is established. The initiator requests chunks ofmemory from the target based on replication requirements or memorywindow thresholds, while the target sends lists of pages to theinitiator via advertising. The initiator and target share positions andsizes of the advertised pool. Persistency and error recovery areachieved via use of three queues (posted, retry, and pending). Errorrecovery steps ensure persistency and absence of data corruption or dataloss.

Before beginning the discussion of the various aspects of theillustrative embodiments, it should first be appreciated that throughoutthis description the term “mechanism” will be used to refer to elementsof the present invention that perform various operations, functions, andthe like. A “mechanism,” as the term is used herein, may be animplementation of the functions or aspects of the illustrativeembodiments in the form of an apparatus, a procedure, or a computerprogram product. In the case of a procedure, the procedure isimplemented by one or more devices, apparatus, computers, dataprocessing systems, or the like. In the case of a computer programproduct, the logic represented by computer code or instructions embodiedin or on the computer program product is executed by one or morehardware devices in order to implement the functionality or perform theoperations associated with the specific “mechanism.” Thus, themechanisms described herein may be implemented as specialized hardware,software executing on general purpose hardware, software instructionsstored on a medium such that the instructions are readily executable byspecialized or general purpose hardware, a procedure or method forexecuting the functions, or a combination of any of the above.

The present description and claims may make use of the terms “a”, “atleast one of”, and “one of” more or with regard to particular featuresand elements of the illustrative embodiments. It should be appreciatedthat these terms and phrases are intended to state that there is atleast one of the particular feature or element present in the particularillustrative embodiment, but that more than one can also be present.That is, these terms/phrases are not intended to limit the descriptionor claims to a single feature/element being present or require that aplurality of such features/elements be present. To the contrary, theseterms/phrases only require at least a single feature/element with thepossibility of a plurality of such features/elements being within thescope of the description and claims.

In addition, it should be appreciated that the following descriptionuses a plurality of various examples for various elements of theillustrative embodiments to further illustrate example implementationsof the illustrative embodiments and to aid in the understanding of themechanisms of the illustrative embodiments. These examples intended tobe non-limiting and are not exhaustive of the various possibilities forimplementing the mechanisms of the illustrative embodiments. It will beapparent to those of ordinary skill in the art in view of the presentdescription that there are many other alternative implementations forthese various elements that may be utilized in addition to, or inreplacement of, the examples provided herein without departing from thespirit and scope of the present invention.

The illustrative embodiments may be utilized in many different types ofdata processing environments. In order to provide a context for thedescription of the specific elements and functionality of theillustrative embodiments. FIGS. 1-3 are provided hereafter as exampleenvironments in which aspects of the illustrative embodiments may beimplemented. It should be appreciated that FIGS. 1-3 are only examplesand are not intended to assert or imply any limitation with regard tothe environments in which aspects or embodiments of the presentinvention may be implemented. Many modifications to the depictedenvironments may be made without departing from the spirit and scope ofthe present invention.

FIG. 1 depicts a pictorial representation of an example storage systemin which aspects of the illustrative embodiments may be implemented. Inthe depicted example, hosts 111, 112 connect to storage system 100 viaFibre Channel (FC) switch 115, and host 113 connects to storage system100 via Internet small computer systems interface (iSCSI) switch 116.Storage system 100 is a scalable enterprise storage system that is basedon a grid array of hardware components. Storage system 100 can attach toboth Fibre Channel Protocol (FCP) and Internet Protocol (IP) networkiSCSI capable hosts 111-113. Storage system 100 supports a wide range ofenterprise features, including hardware redundancy, synchronous andasynchronous mirroring, thin provisioning, data migration, quality ofservice controls, support for virtualization platforms, differentialsnapshots, compression, encryption, etc.

Storage system 100 has a variable number of data modules 101-104 (e.g.,6, 9, 12, or 15 modules). Data modules 101-104 communicate with eachother via an InfiniBand (IB) network, facilitated by two IB switches105, 106 connected via inter-module connectivity 107. Each data module101-104 can serve as a cache driving backend IO and cache coherency. Alimited number of data modules in the depicted example, data modules102, 103—can serve as interfaces, providing FCP or iSCSI connectivity tohosts 111-113.

Storage system 100 may distribute data across all backend storageequally, so that each created volume is striped across all backenddisks. In one example embodiment, two copies of each volume slice may beused, stored on different modules. Each volume slice has a designatedprimary and secondary cache. For reads, the data is fetched by aninterface data module (e.g., data module 102) from the relevant primarycache module (e.g., data module 104). Reading a volume slice will cacheit. For writes, an interface data module (e.g., data module 103) sendsdata to primary cache and the primary cache replicates data to thesecondary cache (e.g., data module 101). The completion of the write isreturned When two copies of data are stored in memory of two differentmodules (e.g., data modules 101, 103). Actual writing of data to backendstorage or eviction to solid state drive (SSD) caches is done in thebackground according to cache management algorithms.

In an alternative embodiment, each volume uses three copies: one primarycopy and two secondary copies. Read IOs fetch relevant data from theprimary cache node and from the backend if not found in cache. The writeIOs send data to primary cache, and the primary cache replicates thedata to the secondary caches. The writes are completed after threecopies of data are stored in memory of three different caches. Thisallows simultaneous loss of two data modules without data loss. At thesame time, rebuilds are significantly faster and require onlysynchronizing caches or cache destages.

As stated above, FIG. 1 is intended as an example, not as anarchitectural limitation for different embodiments of the presentinvention, and therefore, the particular elements shown in FIG. 1 shouldnot be considered limiting with regard to the environments in which theillustrative embodiments of the present invention may be implemented.

As shown in FIG. 1, one or more of the data modules 101-104 and/or hosts111-113 may be specifically configured to implement a mechanism forpersistent memory replication in RDMA-capable networks. The configuringof the computing device may comprise the providing of applicationspecific hardware, firmware, or the like to facilitate the performanceof the operations and generation of the outputs described herein withregard to the illustrative embodiments. The configuring of the computingdevice may also, or alternatively, comprise the providing of softwareapplications stored in one or more storage devices and loaded intomemory of a computing device, such as data module 101 or host 111, forcausing one or more hardware processors of the computing device toexecute the software applications that configure the processors toperform the operations and generate the outputs described herein withregard to the illustrative embodiments. Moreover, any combination ofapplication specific hardware, firmware, software applications executedon hardware, or the like, may be used without departing from the spiritand scope of the illustrative embodiments.

It should be appreciated that once the computing device is configured inone of these ways, the computing device becomes a specialized computingdevice specifically configured to implement the mechanisms of theillustrative embodiments and is not a general purpose computing device.Moreover, as described hereafter, the implementation of the mechanismsof the illustrative embodiments improves the functionality of thecomputing device and provides a useful and concrete result thatfacilitates persistent memory replication.

FIG. 2 is a block diagram of an example data module in which aspects ofthe illustrative embodiments may be implemented. Data module 200comprises processing unit 210, which has a plurality of processing cores201-204, and memory 211. Processing unit 210 connects to peripheralcomponent interconnect express (PCIe) bus 220, through which processingunit 210 communicates with flash cache 212, self-encrypting drive 213,disk drives 221-223, IfiniBand (IB) adapter 230, and IO adapter 240. Inthe depicted example, IB adapter 230 has two IB ports 231, 232, and 10adapter 240 has two Fibre Channel (FC) ports 241, 242 and two Internetsmall computer systems internet (iSCSI) ports 243, 244.

Instructions for the operating system, the object-oriented programmingsystem, and applications or programs are located on storage devices,such as disk drive 221, and may be loaded into memory 215 for executionby processing unit 210. The processes for illustrative embodiments ofthe present invention may be performed by processing unit 210 usingcomputer usable program code, which may be located in a memory such as,for example, memory 215. As such, the data module shown in FIG. 2becomes specifically configured to implement the mechanisms of theillustrative embodiments and specifically configured to perform theoperations and generate the outputs described hereafter with regard topath resolution.

FIG. 3 is a block diagram of just one example data processing system inwhich aspects of the illustrative embodiments may be implemented. Dataprocessing system 300 is an example of a computer, such as host 111 inFIG. 1, in which computer usable code or instructions implementing theprocesses and aspects of the illustrative embodiments of the presentinvention may be located and/or executed so as to achieve the operation,output, and external effects of the illustrative embodiments asdescribed herein.

In the depicted example, data processing system 300 employs a hubarchitecture including north bridge and memory controller hub (NB/MCH)302 and south bridge and input/output (I/O) controller huh (SB/ICH) 304.Processing unit 306, main memory 308, and graphics processor 310 areconnected to NB/MCH 302. Graphics processor 310 may be connected toNB/MCH 302 through an accelerated graphics port (AGP).

In the depicted example, local area network (LAN) adapter 312 connectsto SB/ICH 304. Audio adapter 316, keyboard and mouse adapter 320, modem322, read only memory (ROM) 324, hard disk drive (HDD) 326, CD-ROM drive330, universal serial bus (USB) ports and other communication ports 332,and PCI/PCIe devices 334 connect to SB/ICH 304 through bus 338 and bus340. PCI/PCIe devices may include, for example, Ethernet adapters,add-in cards, and PC cards for notebook computers. PCI uses a card buscontroller, while PCIe does not. ROM 324 may be, for example, a flashbasic input/output system (BIOS).

HDD 326 and CD-ROM drive 330 connect to SB/ICH 304 through bus 340. HDD326 and CD-ROM drive 330 may use, for example, an integrated driveelectronics (IDE) or serial advanced technology attachment (SATA)interface. Super I/O (SIO) device 336 may be connected to SB/ICH 304.

An operating system runs on processing unit 306. The operating systemcoordinates and provides control of various components within the dataprocessing system 300 in FIG. 3. As a client, the operating system maybe a commercially available operating system such as Microsoft® Windows7®. An object-oriented programming system, such as the Java™ programmingsystem, may run in conjunction with the operating system and providescalls to the operating system from Java™ programs or applicationsexecuting on data processing system 300.

As a server, data processing system 300 may be, for example, an IBMeServer™ System p® computer system, Power™ processor based computersystem, or the like, running the Advanced Interactive Executive (AIX®operating system or the LINUX® operating system. Data processing system300 may be a symmetric multiprocessor system including a plurality ofprocessors in processing unit 306. Alternatively, a single processorsystem may be employed.

Instructions for the operating system, the object-oriented programmingsystem, and applications or programs are located on storage devices,such as HDD 326, and may be loaded into main memory 308 for execution byprocessing unit 306. The processes for illustrative embodiments of thepresent invention may be performed by processing unit 306 using computerusable program code, which may be located in a memory such as, forexample, main memory 308, ROM 324, or in one or more peripheral devices326 and 330, for example.

A bus system, such as bus 338 or bus 340 as shown in FIG. 3, may becomprised of one or more buses. Of course, the bus system may beimplemented using any type of communication fabric or architecture thatprovides for a transfer of data between different components or devicesattached to the fabric or architecture. A communication unit, such asmodem 322 or network adapter 312 of FIG. 3, may include one or moredevices used to transmit and receive data. A memory may be, for example,main memory 308, ROM 324, or a cache such as found in NB/MCH 302 in FIG.3.

As mentioned above, in some illustrative embodiments the mechanisms ofthe illustrative embodiments may be implemented as application specifichardware, firmware, or the like, application software stored in astorage device, such as HDD 326 and loaded into memory, such as mainmemory 308, for executed by one or more hardware processors, such asprocessing unit 306, or the like. As such, the computing device shown inFIG. 3 becomes specifically configured to implement the mechanisms ofthe illustrative embodiments and specifically configured to perform theoperations and generate the outputs described hereafter with regard topersistent memory replication.

Those of ordinary skill in the art will appreciate that the hardware inFIGS. 1-3 may vary depending on the implementation. Other internalhardware or peripheral devices, such as flash memory, equivalentnon-volatile memory, or optical disk drives and the like, may be used inaddition to or in place of the hardware depicted in FIGS. 1-3. Also, theprocesses of the illustrative embodiments may be applied to amultiprocessor data processing system, other than the symmetricmultiprocessor system mentioned previously, without departing from thespirit and scope of the present invention.

In accordance with an illustrative embodiment, the storage systemsupports multi-domain interconnect. IO services (cache node, compressionnode, interface node, gateway node) work with multiple worker threads,called “domains.” Each domain makes an effort of working with its ownobjects to minimize locking and inter-thread communication. In someimplementations, single node-to-node connections are established perservice level, and all domains share the same connections. In accordancewith the illustrative embodiment, each domain has a private Interconnectcontext and private connections to other nodes. This has the followingbenefits:

No locking on interconnect objects that is very prohibitive for high IOon non-uniform memory access (NUMA) systems;

Private send and receive queues for each domain that allow resourceoptimization and easy IO processing (post and completions) from domaincontext;

All connectivity and IO events (errors and notifications) are easilyprocessed in the domain context.

Enterprise applications that involve transactions often requiretransaction replication. Examples of such applications are, for example,trading transactions, storage replication (mirroring or storagemigration), or replication of data due to internal storage dataredundancy mechanisms, like implementation of RAID or proprietaryredundancy. These enterprise applications often run on a gridarchitecture (multiple processing nodes) and a remote direct memoryaccess (RDMA) capable interconnect that can perform remote memory writeswithout overhead of the central processing unit (CPU) on local andremote nodes.

While memory replication can be done efficiently using RDMA writes, thequestions of how the remote side is notified about completions, howerror recovery takes place, how remote memory is allocated challengeproprietary solutions.

Use of a standard (e.g., the iSCSI extends for RDMA (iSER) or SCSI RDMAprotocol (SRP)) or proprietary RDMA-capable storage protocol is a commonapproach to this problem. The node that wishes to replicate acts asinitiator, and the remote node acts as target. Transactions are done asfollows:

initiator sends a small write request, providing source addresses

request arrives to target

target allocate memory for data transfer

target does RDMA read to allocated memory

target sends completion status

initiator receives completion status

This answers all questions on memory allocation and error recovery,since these questions are taken care of by the protocol itself, but ithas prohibitive transaction latency. To complete such a transaction, onemust send on initiator and receive on target, followed by RDMA read andsend on target, followed by receive on the initiator. Also, RDMA readshave higher cost then RDMA writes.

Applications that replicate transaction logs often deploy active-passivearchitecture. One node acts as active, and other nodes (one or more) actas standby. When applications start, the memory regions are negotiated,and they are fixed (i.e., writes are done from top to bottom, and memoryis not released to the pool). Transaction logs can be replicated from anactive instance to passive instances using RDMA. If replication fails,the remote node can be considered as failed and can be restarted by thecluster solution, at which point background replication can be done.When an active application fails, some cluster solution selects one ofthe passive instances to assume the active role. The new active instancereads the transaction log and can resume transactions from the correctpoint.

This solution does have low transaction latency, since only RDMA writesare used, but is has limitations:

It has poor error recovery (one failure, and all replication needs to beredone);

It works well for active-passive setups, and does not supportactive-active applications; and,

It works well for applications with journal log transactions. Storageapplications that work with random cache memory that needs to bereplicated and may be released later are not supported well by thissolution.

FIG. 4 is a block diagram illustrating the components of an initiator ina protocol for efficient memory replication using RDMA in accordancewith an illustrative embodiment. Initiator 400 includes initiatorapplication programming interface (API) 410. The initiator API 410includes two main APIs to replicate memory: replicate API 411 and commitAPI 412. Replicate API 411 provides source memory, destination address,context, and notification callback. Commit API 412 provides datastructure destination address, context, and notification callback.Initiator API 410 also provide the following functions: report targetnode connected 413, report target node disconnected 414, and reporttarget node failed 415.

For each replication destination, initiator 400 also includes initiatorRDMA list 420. In order to do RDMA, each initiator maintains a list 420of pages (or agreed size, for example 4 KB), reserved by the target forthis initiator. To request the pages, the initiator sends a page requestmessage. The page request message has the following information: messagecode (page request), serial number of request, tail and free indexpositions in the page pool, minimal number of pages required, andsuggested number of requested pages. This message is sent using thenormal send function and will be received by the target.

The target responds with a page advertise message. The page advertisemessage has the following fields: message code (page advertise), serialnumber of the request, number of pages provided, and array of pageaddresses. This message is sent using the normal send function and willbe received by the initiator. Once the page advertise is received by theinitiator, the list of pages 420 is updated. Initiator RDMA list 420 isan array that contains target addresses. The array is treated as acircular array. Each initiator keeps track of the following index tothat array: head index 411 (index to next free page), tail index 412(index to last free page), and free index 413 (index to last index inthe array that is not populated with any page). When the page advertiseis received by the initiator, the initiator 400 moves the tail index 412forward, which increases the number of available pages in initiator RDMAlist 420. New pages can be added to slots between tail index and freeindex. When an RDMA is attempted, initiator 400 moves the head index 411forward, which decreases the number of available pages in initiator RDMAlist 420. The free index 413 trails the head index. When a commitcompletes, initiator 400 moves free index 413 forward, which increasesthe number of available page slots.

For each replication destination, initiator 400 implements a pool totrack the state of its messages. There are three initiator queues:posted queue 430 contains posted requests, retry queue 440 containsrequests that completed with error, and pending queue 450 containsrequests for which there are no resources in the initiator pool or noRDMA resources in the initiator 400. In one embodiment, the initiatorrequest context provides a node that allows adding to this queue withoutconsuming additional resources.

Because shared domain pools are used for multiple connections andbecause the interconnect might return immediate errors if its queues(shared with other protocols) are full, initiator 400 uses initiatortimers 460 to retry initiator requests. Whenever a timer is requested,initiator 400 does not set a timer if it is already set, if theinitiator is disconnected, or if the posted queue 430 is not empty.

FIG. 5 is a block diagram illustrating the components for a target in aprotocol for efficient memory replication using RDMA in accordance withan illustrative embodiment. Target 500 includes target API 510. Thefollowing functions are available in target API 510: commit callback 511(provides data structure and source address), page request callback 512(provides memory requirements and addresses to fill the results), pageadvertise API 513 (provides memory (list of pages) and number ofprovided pages), report initiator connected 514, report, initiatordisconnected 515, and report initiator failed 516.

Target 500 has the same RDMA array 520 as the initiator to track theinitiator RDMA array picture. The same indexes 521-523 are used and theyare synchronized as messages are processed by the target. In addition tohead index 521, tail index 522, and free index 523, target 500 also usesreported tail index 524. Because target 500 is allowed to advertise withlarger chunks than can fit a single message and because an advertisementmay happen at the target's discretion (without a request from theinitiator), target 500 needs to know what pages have been sent to theinitiator when handling a page request.

Target 500 uses posted queue 530 and retry queue 540, and these queueshave the same functions as they do for the initiator. Because the numberof target requests is small and each target is allowed to have a finitenumber of page advertisements, the size of the request pool is finiteand cannot be exhausted.

Like the initiator, target 500 may use retry timers 560, becauseinterconnect queues may share multiple protocols and become full.

FIGS. 6A and 6B are flowcharts illustrating operation of an initiatorperforming an RDMA request in accordance with an illustrativeembodiment. With reference to FIG. 6A, operation begins (block 600), andthe initiator determines whether the target node is blacklisted (block601). If the target node is blacklisted, then the initiator callsapplication completion callback that indicates immediate failure (block602), and operation ends (block 603).

If the target is not blacklisted in block 601, then the initiatordetermines whether the pending queue or the retry queue are not empty(block 604). If the pending or retry queue are not empty, the initiatoradds the RDMA request to the pending queue (block 605), and operationends (block 603).

If both the pending queue and retry queue are empty in block 604, theinitiator determines whether the target is disconnected (block 606). Ifthe target is disconnected, the initiator adds the RDMA request to thepending queue (block 605), and operation ends (block 603).

If the target is not disconnected in block 606, then the initiatordetermines whether there is enough RDMA memory (bock 607). If there isnot enough RDMA memory, the initiator adds the RDMA request to thepending queue (block 608) and sends the page request to the target(block 609). Thereafter, operation ends (block 603).

If there is enough RDMA memory in block 607, the initiator allocates arequest context (block 610). Then, the initiator determines whetherallocating the request context failed (block 611). If allocating therequest context failed, the initiator adds the RDMA request to thepending queue (block 612) and configures a retry timer (block 613).Thereafter, operation ends (block 603).

If the initiator determines that allocating the request context succeedsin block 611, the initiator adds the request parameters to the requestcontext (block 614), posts the RDMA context to the RDMA request (block615), and determines whether the posting failed (block 616). If theposting failed, the initiator adds the RDMA request to the retry queue(block 617). Then, the initiator determines whether the posting failedwith the queue size full code (block 618). If the posting failed withthe queue size full code, then the initiator configures a retry timer(block 613), and operation ends (block 603). If the initiator determinesthe posting did not fail with the queue size full code in block 618,then operation ends (block 603).

If the initiator determines the posting succeed in block 616, thenoperation proceeds to block 619 in FIG. 6B, and the initiator adds theRDMA request to the posted list. Then, the initiator determines whetherthe RDMA pool is low (block 620). If the RDMA pool is low, then theinitiator posts a page request (block 621), and operation ends (block622). If the initiator determines the RDMA pool is not low in block 620,then operation ends (block 622). If the number of available page slots(not populated with any page) is low, the application may delay sendingpage request until more free slots become available (slots becomeavailable with the completion of initiator transfers). This allows tonot send page requests too frequently, since small page requests aresufficient for small number of requests.

FIG. 7 is a flowchart illustrating operation of an initiator performinga commit request in accordance with an illustrative embodiment. It isexpected that commit messages follow the RDMA message. They are expectedto be sent with the same input/output sequence number (IOSN) as thetracked RDMA message to which they correspond. The reason this exists asan API is to allow the initiator to coalesce several commit messagesinto one message. If no coalescing is used, then each RDMA request isfollowed by a commit request.

The commit message has the following information: message code (commitrequest), start index position in initiator pool for this commitrequest, number of pages, free index position in the initiator pool,commit data structure, and serial number of request. This message issent using the normal send function and will be received by the target.

Operation begins (block 700), and the initiator determines whether thetarget node is blacklisted (block 701). If the target node isblacklisted, then the initiator calls the application callback withimmediate failure status (block 702) Then, operation ends (block 703).

If the target node is not blacklisted in block 701, then the initiatordetermines whether the pending queue or the retry queue are not empty(block 704). If the pending or retry queue are not empty, the initiatoradds the commit request to the pending queue (block 705), and operationends (block 703).

If both the pending queue and retry queue are empty in block 704, theinitiator determines whether the target is disconnected (block 706). Ifthe target is disconnected, the initiator adds the commit request to thepending queue (block 705), and operation ends (block 703).

If the target is not disconnected in block 706, then the initiatorallocates a request context (block 707). Then, the initiator determineswhether allocating the request context failed (block 708). If allocatingthe request context failed, the initiator adds the commit request to thepending queue (block 709) and configures a retry timer (block 710).Thereafter, operation ends (block 703).

If the initiator determines that allocating the request context succeedsin block 708, the initiator adds the request parameters to the requestcontext (block 711), posts the commit request (block 712), anddetermines whether the posting failed (block 713). If the postingfailed, the initiator adds the commit request to the retry queue (block714). Then, the initiator determines whether the posting failed with thequeue size full code (block 715). If the posting failed with the queuesize full code, then the initiator configures a retry timer (block 710),and operation ends (block 703). If the initiator determines the postingdid not fail with the queue size full code in block 713, then operationends (block 703).

If the initiator determines the posting succeed in block 713, then theinitiator adds the commit request to the posted list (block 716). Then,operation ends (block 703).

FIG. 8 is a flowchart illustrating operation of the initiator performinga page request in accordance with an illustrative embodiment. Only oneunanswered pending page request is allowed per target (replicationdestination).

Operation begins (block 800), when initiator requests a variable minimalnumber of pages. The minimal number of pages will be zero if pagerequest is due to refill of the pool and non-zero is page request is dueto replication request that cannot be satisfied because the lack ofpages. At branch 801, the initiator determines whether the PAGE REQUESTflag is set (whether there is unanswered page request). If the flag isset (block 801: YES), the operation ends (block 802). If the PAGEREQUEST flag is not set (block 801: NO), operation continues at block803, where the initiator computes the number of free slots.

The operation then proceeds to branch 804, where the amount of freeslots is compared with the minimal number of requested pages. If thenumber of free slots is less than the minimal number of requested pages(block 803: YES), then operation ends (block 802). Otherwise, theoperation proceeds to branch 805, where the initiator determines whetherthe request is due to refill (minimal number of required pages is zero).If the request is not due to refill (block 805: NO), the operationproceeds to block 806, where the initiator allocates a request context.Because no initiator is allowed to have more than one pending pagerequest, page requests can use a private request pool of a finite size,the allocation from which cannot fail. The initiator then fills therequest parameters (block 807), sets the page request flag (block 808),and posts the page request message (block 809).

A page request message has the following information: message code (pagerequest), serial number of request, tail and free index positions in thepage pool, minimal number of pages required, and suggested number ofprovided pages. The suggested number of provided pages is the minimum offree pages and MAX_PAGE_REQ_SIZE a protocol constant that defines themaximal number of pages that can be received due to a single pagerequest. This message is sent using the normal send function and will bereceived by target. Then, the initiator determines whether the postingfailed (block 810). If the initiator determines the posting succeeded,the initiator adds the page request to the posted queue (bock 811).Thereafter, operation ends (block 802).

If the initiator determines the posting failed in block 810, theinitiator adds the page request to the retry queue (block 812). Then,the initiator determines whether the posting failed with theinterconnect queues full code (block 813). If the posting failed withthe interconnect queues full code, then the initiator configures a retrytimer (block 814), and operation ends (block 802). If the initiatordetermines the posting did not fail with the interconnect queues fallcode in block 813, then operation ends (block 802).

Returning to block 805, if the request is due to refill, then theinitiator determines whether enough free slots are available to justifyimmediate send of the page request (block 815). To minimize the amountof page requests, it may be desirable to send refill requests whencommit completions arrive and more free slots become available. Toachieve this, refill page requests are not sent if the amount of freeslots is less than MIN_PAGE_REQ_SIZE . . . a predefined protocolconstant. If the amount of free slots is less than the definedthreshold, meaning there are not enough free slots available to justifysending a page request (block 805: NO), then operation ends (block 802).Otherwise, operation proceeds to blocks 806-814, where the initiatorimmediately posts a page request.

FIGS. 9A-9C are flowcharts illustrating the operations of mechanisms forhandling completions in accordance with an illustrative embodiment. Morespecifically, FIG. 9A is a flowchart illustrating operation of amechanism for handling RDMA completion in accordance with anillustrative embodiment. Operation begins (block 900), and the mechanismdetermines whether the completion indicates an error (block 901). If thecompletion indicates an error, then the mechanism moves all requestsfrom the posted queue to the retry queue (block 902) and marks theinitiator as disconnected (block 903). (Requests on RDMA capable devicescomplete in the same order as they were posted, and a first completionerror means the target is disconnected and also that the rest of thecompletions will return error.) Thereafter, operation ends (block 904).

If the mechanism determines the completion does not indicate an error inblock 901, then the mechanism removes the RDMA request context from theposted queue (block 905), invokes the initiator callback (block 906),frees the request context (block 907), and resumes the initiator (block908). Resume takes place if either retry or pending lists were not emptydue to shortage of resources in protocol pools or interconnect queuesthat have just become available. Thereafter, operation ends (block 904).

FIG. 9B is a flowchart illustrating operation of a mechanism forhandling commit completion in accordance with an illustrativeembodiment. Operation begins (block 910), and the mechanism determineswhether the completion indicates an error (block 911). If the completionindicates an error, then the mechanism moves the requests from theposted queue to the retry queue (block 912) and marks the initiator asdisconnected (block 913). (Post requests on RDMA capable devicescomplete in the same order as they were posted, and a first completionerror means target is disconnected and it also means the rest of thecompletions will return error.) Thereafter, operation ends (block 914).

If the mechanism determines the completion does not indicate an error inblock 911, then the mechanism removes the request context from theposted queue (block 915), invokes the initiator callback (block 916),and frees the request context (block 917). The mechanism also updatesthe free index of the RDMA page array to reflect the commit messageparameters (block 918). At branch 919, the initiator checks whetherrefill of the page pool is justified. Refill of the page pool isjustified if the free pages is below threshold and the amount of freepages is no less than MIN_PAGE_REQ_SIZE value described above. If pagerefill is justified (block 919: YES), the page request is sent (block920) and operation proceeds to block 921. If page request is notjustified (block 919: NO), then operation proceeds to block 921.

At block 921, the initiator is resumed. Resume takes place if eitherretry or pending lists were not empty due to shortage of resources inprotocol pools or interconnect queues that have just become available.Thereafter, operation ends (block 914).

FIG. 9C is a flowchart illustrating operation of a mechanism forhandling page request completion in accordance with an illustrativeembodiment. Operation begins (block 930), and the mechanism determineswhether the completion indicates an error (block 931). If the completionindicates an error, then the mechanism moves requests from the postedqueue to the retry queue (block 932) and marks the initiator asdisconnected (block 933). (Post requests on RDMA capable devicescomplete in the same order as they were posted, and a first completionerror means target is disconnected and it also means the rest of thecompletions will return error.) Thereafter, operation ends (block 934).

If the mechanism determines the completion does not indicate an error inblock 931, then the mechanism removes the request context from theposted queue (block 935), frees the request context (block 936), andresumes the initiator (block 937). Resume takes place if either retry orpending lists were not empty due to shortage of resources in protocolpools or interconnect queues that have just become available.Thereafter, operation ends (block 934).

FIG. 10 is a flowchart illustrating operation of a mechanism forhandling target page advertise messages in accordance with anillustrative embodiment. Operation begins (block 1000), and themechanism determines whether the peer (target) IOSN (IO serial number)value of the request is an unexpected value (block 1001). The initiatormay track target IOSN numbers to detect protocol errors. If themechanism determines the target IOSN value is unexpected, then themechanism determines this is a fatal application error and rejects themessage (block 1002). Thereafter, operation ends (block 1003).

If the target IOSN value is not an unexpected value in block 1001, thenthe mechanism determines whether the number of free slots in the RDMAarray is less than the number of provided pages (block 1004). If thenumber of free slots in the RDMA array is less than the number ofprovided pages, then the mechanism determines this is a finalapplication error and rejects the message (block 1002). Thereafter,operation ends (block 1003).

If the number of free slots in the RDMA array is not less than thenumber of provided pages in block 1004, then the mechanism incrementsthe target IOSN index (block 1005), updates the RDMA page array with theaddresses provided in the message (block 1006), updates the tail index(block 1007), and clears the page request flag (block 1008). Then, themechanism calls resume of the initiator (block 1009), and operation ends(block 1003). Resume will take place if retry or pending lists are notempty due to shortage of resources that have just become available.Specifically, in the case of advertised pages there may be pendingrequests that could not be satisfied because pages were not available.

FIG. 11 is a flowchart illustrating operation of a mechanism for targethandling of a page advertisement application request in accordance withan illustrative embodiment. Each target is allowed a finite number ofpage advertisements pending for a specific initiator (called pageadvertised credits). This is due to the fact that the target may needseveral messages to populate the entire RDMA array. The size of a singleadvertise message may be limited and not sufficient to populate theentire array. At the same time, the size of a pool for page advertisecontext is finite, and we want to insure that request contextallocations do not fail. The credits allocated for each initiator takethese constraints into account.

Operation begins when a page advertisement API is invoked (block 1100),and the mechanism determines whether there are enough free slots tostore the provided pages (block 1101). If there are not enough treeslots, the mechanism returns extra pages to the target with a releasecallback (block 1102). Thereafter, operation continues at block 1104.

If there are enough free slots in block 1101, then the mechanismpopulates the RDMA array (block 1104) and updates the tail index (bock1105). Then, the mechanism determines whether the page advertise creditis zero (block 1106). At the start of the application, the credit isdefined to a predefined number, and each queued page advertisedecrements the credit value while each completion of a page advertiseincrements the credit value. If the page advertise credit is zero, thenthe mechanism sets the NEED_PAGE_ADVERTISE flag (block 1107).Thereafter, operation ends (block 1103).

If the page advertise credit is not zero in block 1106, then themechanism decrements the credit (block 1108), allocates a requestcontext (block 1109), fills the request context (block 1110), and poststhe advertise message (block 1111). Because the interconnect works withfinite size buffers on the receiver side of the initiator, the mechanismcannot advertise more addresses than fit into that buffer. The mechanismupdates the “reported tail” index of the RDMA array to reflect thepending page advertisement (block 1112).

Then, the mechanism determines whether the posting message is successful(block 1113). If the posting is successful, the mechanism adds therequest context to the posted queue (block 1114), and then the mechanismdetermines whether the reported tail index equals the tail index (block1115). If the reported tail index is not equal to the tail index, thenoperation returns to block 1106, and blocks 1106-1115 repeat until thereported tail index equals the tail index. If the reported tail indexdoes equal the tail index in block 1115, then operation ends (block1103).

If the posting is not successful in block 1113, then the mechanism addsthe request to the retry queue (block 1116). The mechanism determineswhether the posting failed with the interconnect queues full code (block1117). If the posting failed with the interconnect queues full code,then the mechanism configures a retry timer (block 1118), and operationends (block 1103), if the mechanism determines that the posting did notfail with the interconnect queues full code in block 1117, thenoperation ends (block 1103).

FIG. 12 is a flowchart illustrating operation of a mechanism for targethandling of a page request in accordance with an illustrativeembodiment. Operation begins with the receipt of a page request (block1200), and the mechanism determines whether the peer (initiator) ISONvalue of the request is an unexpected value (block 1201). The target maytrack the initiator IOSN values to detect protocol violations. If themechanism determines the initiator IOSN value is unexpected, then themechanism rejects the message (block 1202). Thereafter, operation ends(block 1203).

If the initiator IOSN value is not an unexpected value in block 1201,then the mechanism increments the initiator IOSN index on the target(block 1204). At step 1205, the mechanism determines the number of pagesin-flight. The mechanism checks the tail index of the request with thereported tail index. The difference between them is the number of pagesin flight that, the target has sent but the initiator has not yetreceived. It is possible that this number is not zero if the initiatorhas sent this page request before processing the in-flight pageadvertise message. At branch 1206, the mechanism determines whether thenumber of pages in flight is not less than the maximal pages specifiedin the request. If yes (the previous page advertise from target hasalready satisfied this request), then operation ends (block 1203). Ifno, then operation proceeds to block 1207, where the mechanism computesthe number of non-sent pages. The number of non-sent pages is thedifference between the tail index and reported tail index on target. Itis possible that this number is not zero, if the target application haspreviously tried to advertised pages, but page advertisements could notbe sent due to lack of credits.

At block 1208, the mechanism recomputes the minimal and maximal numberof pages in the initiator request. The minimal and maximal number aredecremented by the number of pages in flight but are not allowed to gobelow zero. Furthermore, the maximal number is not allowed to be largerthan the amount of non-sent pages plus the amount of free page slots. Inaddition, if the number of free pages is below a defined threshold, themaximal number of pages is set to the amount of free page slots (fullarray refill is requested). At branch 1209, the mechanism determineswhether the number of non-sent pages is not less than the recomputedmaximal number of pages. If yes, then the mechanism calls the pageadvertise function (block 1210), and operation ends (block 1203). Thepage advertise function starts from block 1106 of FIG. 11.

If non-sent pages number does not satisfy the page request (block 1209:NO), the operation proceeds to block 1211, where the page advertisecredit is recorded. The mechanism then computes the missing pages numberas the difference between recomputed maximal number and non-sent pages(block 1212). Then, the mechanism requests pages from the targetapplication (block 1213). The minimal number of pages in the request isthe recomputed minimal number, and the maximal number of pages is themissing pages number. At branch 1214, the mechanism checks whether thepage advertise credit has changed. If credit has changed (target hasprovided pages and has invoked page advertise function), then operationends (block 1203). If credit has not changed (block 1214: NO), themechanism determines whether the number of non-sent pages is zero. Ifthe number of non-sent pages is zero, then operation ends (block 1203).If the number of non-sent pages is not zero (pages to advertise areavailable and target application has not added new pages or could notsend because lack of credits), then operation proceeds to block 1210,where page advertise function is called, and operation ends (block1203).

FIG. 13 is a flowchart illustrating operation of a mechanism for targethandling of a commit message in accordance with an illustrativeembodiment. Operation begins with the receipt of a commit message (block1300), and the mechanism determines whether the peer (initiator) ISONvalue of the request is an unexpected value (block 1301). The target maytrack initiator IOSN values to detect protocol violations. If themechanism determines the IOSN value is unexpected, then the mechanismrejects the message (block 1302). Thereafter, operation ends (block1303).

If the IOSN value is not an unexpected value in block 1301, then themechanism increments the peer (initiator) IOSN index on the target(block 1304). For aggregated messages, the IOSN is incremented by morethan one. The mechanism updates the free index of the RDMA array to thevalue in the request (block 1305). The mechanism updates the head indexof the RDMA array to the start index of the request plus the page countnumber of the request (block 1306). The mechanism then calls the targetcommit callback (block 1307), and operation ends (block 1303).

FIG. 14 is a flowchart illustrating operation of a mechanism for targethandling of a page advertise completion in accordance with anillustrative embodiment. Operation begins with the receipt of a pageadvertise request (block 1400), and the mechanism determines whether thecompletion indicates success (block 1401). If the completion indicatessuccess, the mechanism removes the request from the posted queue andreleases the request context (block 1402). The mechanism increments thepage advertise credit value (block 1403). The mechanism then checkswhether NEED_PAGE_ADVERTISE flag is set (branch 1404). If yes, the pageadvertise function (from FIG. 11) is called (block 1405). Thereafter,operation ends (block 1406). If NEED_PAGE_ADVERTISE flag is not set, theoperation proceeds to end (block 1406).

If the completion does not indicate success in block 1401, then themechanism moves all requests from the posted queue to the retry queue(block 1407) and marks the target as disconnected (block 1408).Thereafter, operation ends (block 1406).

FIG. 15 is a flowchart illustrating operation of a mechanism fordisconnect handling in accordance with an illustrative embodiment.Operation begins when an initiator or target disconnects (block 1500)The mechanism clears the receive queue (block 1501). Both the initiatorand the target shall clear its receive queue before resetting connectionor they risk receiving with error transactions seen by the peer ascompleted. To do so, the mechanism polls the receive queue until itsqueue depth is reached or until no new completions are seen, whichevercomes first. After that, the mechanism resets the connection (block1502), and marks the state of the initiator or target as disconnected(bock 1503). Thereafter, operation ends (block 1504).

FIG. 16 is a flowchart illustrating operation of a mechanism forhandling target dead node event by the initiator in accordance with anillustrative embodiment. Operation begins when a dead node event isreceived (block 1600). The posted queue is expected to be empty, becauseall disconnect handling should have been completed at this point. Themechanism calls callbacks of all requests from the retry and pendingqueues with error status (NOT_SENT) (block 1601). The mechanism releasesthe request contexts of the retry queue (block 1602). Then, themechanism empties retry and pending queues (block 1603). The mechanismmarks the initiator status as flushed (meaning the target node isblacklisted) and disconnected (block 1604). The mechanism then resetsall indexes of the RDMA array to zero (block 1605), and operation ends(block 1606).

FIG. 17 is a flowchart illustrating operation of a mechanism forhandling initiator dead node event by target in accordance with anillustrative embodiment. Operation begins when a dead node event isreceived (block 1700). The posted queue is expected to be empty, becauseall disconnect handling should have been completed at this point. Themechanism removes all requests from the retry queue and releases requestcontexts (block 1701). The mechanism calls target callback to free pagesfrom the RDMA array (block 1702) and marks the target status as flushed(meaning the initiator node is blacklisted) (block 1703). The mechanismthen resets all indexes of the RDMA array to zero (block 1704), andoperation ends (block 1705).

FIG. 18 is a flowchart illustrating operation of mechanism for initiatorand target resume in accordance with an illustrative embodiment.Initiator and target resume can be called from a connection event (whena previously broken connection resumes and login completes) or from aretry timer or successful completion When previously unavailableresources become available. Operation begins (block 1800), where themechanism reposts the first message from the retry queue (block 1801).At branch 1802, the post status is checked. If the mechanism determinesthat there was a post error (block 1802: YES), then the mechanismdetermines whether the failure was due to the shortage of interconnectbuffers (block 1803). If the failure was due to the shortage ofinterconnect buffers, then the mechanism configures a retry timer (block1804). Thereafter, or if the mechanism determines that the failure wasnot due to a shortage of interconnect buffers, the resume aborts (block1805).

If there is no post error (block 1802: NO), then the mechanism removesthe successfully posted message from the retry queue and adds it to theposted queue (block 1806). The mechanism determines whether the retryqueue is empty (block 1807). If the retry queue is not empty, thenoperation returns to block 1801 to report the next message from theretry queue.

If the retry queue is empty in block 1807, the mechanism takes the firstmessage from the pending queue and treats it as a new initiator ortarget request (block 1808). This step removes the message from thepending queue and invokes standard request functions that can modify theposted and retry queues, with the exception that if there are no localresources to frame the request, it will not be added to pending queuebut allocation failure will be returned. The mechanism determineswhether there is a request process error (branch 1809). If there is arequest process error, the error status is examined to be allocationfailure (branch 1810). If the error is allocation error, the request isadded to the top of the pending list (block 1811) and operation aborts(step 1805). If the error is not an allocation error but a post error,the request has been added to the retry list, and operation aborts (step1805).

If there is not a request process error block 1809, the mechanismdetermines whether the pending queue is empty (block 1812). If thepending queue is not empty, then operation returns to block 1808 to takethe next message from the pending queue. If the pending queue is emptyin block 1812, then operation ends (block 1813).

Both initiator and target rely on local completions to conclude that thepeer has received the message. This is true for successful completions(provided the peer clears receive queue before it resets theconnection). However, this is not true for completions received witherror. The local error completion does not mean the peer has notreceived the transaction. This can happen during a loss of the physicalwhen peer sees the message but cannot acknowledge it using the physicallayer because the link has gone down. Or it may happen because the peerwas very busy and has not acknowledged completion in time using thephysical layer, causing a peer timeout error. In order to avoid the datacorruption on the peer after reconnect (by resending the data that hasbeen received and used) a synchronization message is needed that willinform the peer about the last IOSN received. The login message solvesthis task. The login message has only the message code (login).

FIGS. 19A and 19B are flowcharts illustrating operation of a mechanismfor sending a login message from connection event in accordance with anillustrative embodiment. Handling of login messages is the same for theinitiator and the target. Operation begins (block 1900), upontransitioning to a connected state the mechanism determines whether theretry queue is empty (block 1901). If the retry queue is empty, thenoperation ends (block 1902). If the retry queue is not empty, themechanism sets the LOGIN_REQUESTED flag (block 1903). Once theLOGIN_REQUESTED flag is set, no resume or post is allowed. All new postsgo to the pending queue. Thereafter, the mechanism invokes the resendlogin message function (block 1904 see FIG. 19B), and operation ends(block 1902).

FIG. 19B illustrates the resend login function. Operation begins atblock 1910. At block 1911, the login message is framed and sent. Loginmessage requires no allocated context and has only the message opcodeand the IOSN number. No completion is requested for login message, andno protocol callback is invoked for it. The mechanism then determineswhether the post is successful (block 1912). If the post is successful,the mechanism clears the LOGIN_NEEDED flag (block 1913), and operationends (block 1914). If the mechanism determines the post is notsuccessful and completes with error in block 1912, the mechanismexamines the error code is retriable (block 1915). The error isretriable when error indicated that queues are full on interconnect). Ifthe error is retriable, then the mechanism sets LOGIN_NEEDED flag (block1916) and configures a retry timer (block 1917). Then, operation ends(block 1914). If the error is not retriable (i.e., the peer hasdisconnected), operation clears LOGIN_NEEDED and LOGIN_REQUESTED flags(block 1918), sets disconnected flag (block 1919), and ends (block1914).

FIG. 20 is a flowchart illustrating operation of a mechanism forretrying login from the timer in accordance with an illustrativeembodiment. Operation begins (block 2000), and the mechanism determineswhether the LOGIN_NEEDED flag is set (block 2001). If the LOGIN_NEEDEDflag is not set, then operation ends (block 2003). If the LOGIN_NEEDEDflag is set in block 2001, then the mechanism resends the login message(block 2002, see FIG. 19B), and operation ends (block 2003).

FIG. 21 is a flowchart illustrating operation of a mechanism forhandling login message response in accordance with an illustrativeembodiment. Operation beings upon receiving the login message response(block 2100), and the mechanism examines the retry queue (block 2101).The mechanism removes all entries with IOSN less than or equal to thevalue in the response from the retry queue and considers them tocomplete with OK status (block 2102). On the initiator, the mechanismcalls appropriate callbacks (block 2103). Then, the mechanism clearsLOGIN_REQUESTED flag (block 2104), resumes the instance (initiator ortarget) (block 2105), and operation ends (block 2106).

FIG. 22 is a flowchart illustrating operation of a mechanism forhandling a login message request in accordance with an illustrativeembodiment. Operation begins upon receiving a login message request(block 2200), and the mechanism sets the NEED_LOGIN_REPLY flag (block2201). The mechanism frames and posts the reply (block 2202). The loginresponse message has the following fields: message code (login response)and the last peer IOSN that has been processed by the receiver. Nocontext is allocated for the message, but the completion is requested.The mechanism determines whether the post completes with error (block2203). If the post completes without error, then the mechanism sets theLOGIN_REPLY_POSTED flag (block 2204), clears NEED_LOGIN_REPLY flag(block 2205), and operation ends (block 2206). If the post completeswith error in block 2203, then the mechanism examines the error code(block 2207) and determines whether the error is retriable (block 2208).If the error is not retriable, the mechanism clears the NEED_LOGIN_REPLYflag (block 2209), sets DISCONNECTED flag (block 2210), and thenoperation ends (block 2206), if the error is retriable, then themechanism configures a retry timer (block 2211), and operation ends(block 2206).

FIG. 23 is a flowchart illustrating operation of a mechanism forretrying login response from a timer in accordance with an illustrativeembodiment. Operation begins (block 2300), and the mechanism determineswhether the NEED_LOGIN_REPLY flag is set (block 2301). If theNEED_LOGIN_REPLY flag is not set in block 2301, then operation ends(block 2303). If the NEED_LOGIN_REPLY flag is set in block 2301, thenthe mechanism resends the login response message (block 2302, see FIG.22), and operation ends (block 2304). The login response message has thefollowing fields: message code (login response) and last initiator IOSNthat completed without error. The login response message is sent usingthe normal send function and will be received by the target.

FIG. 24 is a flowchart illustrating operation of a mechanism forhandling a login response completion in accordance with an illustrativeembodiment. Operation begins upon receiving a login response completion(block 2400), and the mechanism removes the LOGIN_REPLY_POSTED flag(block 2401). The mechanism determines whether the login reply completedsuccessfully (block 2402). If the login reply completed successfully,then operation ends (block 2403). If the login reply did not completesuccessfully, then the DISCONNECT and NEED_LOGIN reply flags are set(block 2404) operation ends (block 2403). Upon completion failure,NEED_LOGIN_REPLY flag is set. This is because initiator and target mayuse half-duplex connections (like XRC), and disconnect of local nodedoes not mean that remote node has lost connection. When connection isresumed, the NEED_LOGIN_REPLY flag is checked. If it is set, the localnode will resend login reply, without receiving the peer login request(that may not arrive, since the peer may have not disconnected).

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Java, Smalltalk, C++ or the like,and conventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

As noted above, it should be appreciated that the illustrativeembodiments may take the form of an entirely hardware embodiment, anentirely software embodiment or an embodiment containing both hardwareand software elements. In one example embodiment, the mechanisms of theillustrative embodiments are implemented in software or program code,which includes but is not limited to firmware, resident software,microcode, etc.

A data processing system suitable for storing and/or executing programcode will include at least one processor coupled directly or indirectlyto memory elements through a communication bus, such as a system bus,for example. The memory elements can include local memory employedduring actual execution of the program code, bulk storage, and cachememories which provide temporary storage of at least some program codein order to reduce the number of times code must be retrieved from bulkstorage during execution. The memory may be of various types including,but not limited to, ROM, PROM, EPROM, EEPROM, DRAM, SRAM, Flash memory,solid state memory, and the like.

Input/output or I/O devices (including but not limited to keyboards,displays, pointing devices, etc.) can be coupled to the system eitherdirectly or through intervening wired or wireless I/O interfaces and/orcontrollers, or the like. I/O devices may take many different formsother than conventional keyboards, displays, pointing devices, and thelike, such as for example communication devices coupled through wired orwireless connections including, but not limited to, smart phones, tabletcomputers, touch screen devices, voice recognition devices, and thelike. Any known or later developed I/O device is intended to be withinthe scope of the illustrative embodiments.

Network adapters may also be coupled to the system to enable the dataprocessing system to become coupled to other data processing systems orremote printers or storage devices through intervening private or publicnetworks. Moderns, cable modems and Ethernet cards are just a few of thecurrently available types of network adapters for wired communications.Wireless communication based network adapters may also be utilizedincluding, but not limited to, 802.11 a/b/g/n wireless communicationadapters, Bluetooth wireless adapters, and the like. Any known or laterdeveloped network adapters are intended to be within the spirit andscope of the present invention.

The description of the present invention has been presented for purposesof illustration and description, and is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the describedembodiments. The embodiment was chosen and described in order to bestexplain the principles of the invention, the practical application, andto enable others of ordinary skill in the art to understand theinvention for various embodiments with various modifications as aresuited to the particular use contemplated. The terminology used hereinwas chosen to best explain the principles of the embodiments, thepractical application or technical improvement over technologies foundin the marketplace, or to enable others of ordinary skill in the art tounderstand the embodiments disclosed herein.

What is claimed is:
 1. A method, in a data processing system comprisingat least one processor and at least one memory, the at least one memorycomprising instructions that are executed by the at least one processorand configure the at least one processor to implement a replicationprotocol for replicating memory from an initiator to a target, themethod comprising: requesting, by the initiator, one or more pages ofmemory at the target; responsive to the initiator receiving a pageadvertisement from the target, updating a list of pages at theinitiator, wherein the list of pages is replicated at the target;performing, by the initiator, a remote direct memory access (RDMA) writeto the one or more pages of memory at the target; responsive tosuccessful completion of the RDMA write, updating the list of pages atthe initiator, wherein upon completion of the RDMA write the list ofpages is updated at the target; responsive to disconnecting from thetarget and subsequently reconnecting to the target, receiving a loginmessage from the target; sending a login reply message to the target,wherein the login reply message comprises a message code and a lastinitiator input/output sequence number (IOSN) that completed withouterror; and responsive to receiving a login reply message from the targetcomprising a last target input/output initiator sequence number (IOSN)processed by target, removing all messages from a retry queue having anIOSN less than or equal to the last IOSN reported as processed by targetand resuming the initiator.
 2. The method of claim 1, furthercomprising, responsive to successful completion of the RDMA write,sending a commit message from the initiator to the target.
 3. The methodof claim 1, wherein the list of pages at the initiator is a circularqueue, wherein a head index points to a next free page in an RDMA poolat the initiator, wherein a tail index points to a last free page in theRDMA pool at the initiator, and wherein a free index points to a lastindex in the RDMA pool that is not populated with any page.
 4. Themethod of claim 1, wherein performing the RDMA write comprises sendingan RDMA request from the initiator to the target and placing the RDMArequest in a posted queue at the initiator and wherein performing theRDMA write further comprises adding the RDMA request to a pending queueat the initiator responsive to determining the pending queue or a retryqueue at the initiator is not empty, determining the target isdisconnected, or determining there are not enough memory pages for theRDMA request at the target.
 5. The method of claim 1, wherein performingthe RDMA write comprises sending an RDMA request from the initiator tothe target and placing the RDMA request in a posted queue at theinitiator and wherein performing the RDMA write further comprises addingthe RDMA request to a retry queue at the initiator responsive todetermining that the RDMA write failed.
 6. The method of claim 1,wherein performing the RDMA write comprises sending an RDMA request fromthe initiator to the target and placing the RDMA request in a postedqueue at the initiator and wherein performing the RDMA write furthercomprises adding the RDMA request to a posted list at the initiatorresponsive to determining that the RDMA write succeeded.
 7. The methodof claim 1, wherein performing the RDMA write comprises sending an RDMArequest from the initiator to the target and placing the RDMA request ina posted queue at the initiator and wherein performing the RDMA writefurther comprises sending a commit message responsive to successfulcompletion of the RDMA write.
 8. The method of claim 1, wherein theinitiator and the target use input/output sequence numbers (IOSNs) totrack messages.
 9. The method of claim 1, further comprising: responsiveto determining the target is blacklisted, calling an applicationcallback with immediate failure status.
 10. A computer program productcomprising a non-transitory computer readable storage medium having acomputer readable program stored therein, wherein the computer readableprogram, when executed on a computing device, causes the computingdevice to implement a replication protocol for replicating memory froman initiator to a target, wherein the computer readable program causesthe computing device to: request, by the initiator, one or more pages ofmemory at the target; responsive to the initiator receiving a pageadvertisement from the target, update a list of pages at the initiator,wherein the list of pages is replicated at the target; perform, by theinitiator, a remote direct memory access (RDMA) write to the one or morepages of memory at the target; responsive to successful completion ofthe RDMA write, update the list of pages at the initiator, wherein uponcompletion of the RDMA write the list of pages is updated at the target;responsive to disconnecting from the target and subsequentlyreconnecting to the target, receive a login message from the target;send a login reply message to the target, wherein the login replymessage comprises a message code and a last initiator input/outputsequence number (IOSN) that completed without error; and responsive toreceiving a login reply message from the target comprising a last targetinput/output initiator sequence number (IOSN) processed by target,remove all messages from a retry queue having an IOSN less than or equalto the last IOSN reported as processed by target and resume theinitiator.
 11. The computer program product of claim 10, wherein thecomputer readable program further causes the computing device to,responsive to successful completion of the RDMA write, sending a commitmessage from the initiator to the target.
 12. The computer programproduct of claim 10, wherein the list of pages at the initiator is acircular queue, wherein a head index points to a next free page in anRDMA pool at the initiator, wherein a tail index points to a last freepage in the RDMA pool at the initiator, and wherein a free index pointsto a last index in the RDMA pool that is not populated with any page.13. The computer program product of claim 10, wherein performing theRDMA write comprises sending an RDMA request from the initiator to thetarget and placing the RDMA request in a posted queue at the initiatorand wherein performing the RDMA write further comprises adding the RDMArequest to a pending queue at the initiator responsive to determiningthe pending queue or a retry queue at the initiator is not empty,determining the target is disconnected, or determining there are notenough memory pages for the RDMA request at the target.
 14. The computerprogram product of claim 10, wherein performing the RDMA write comprisessending an RDMA request from the initiator to the target and placing theRDMA request in a posted queue at the initiator and wherein performingthe RDMA write further comprises adding the RDMA request to a retryqueue at the initiator responsive to determining that the RDMA writefailed.
 15. The computer program product of claim 10, wherein performingthe RDMA write comprises sending an RDMA request from the initiator tothe target and placing the RDMA request in a posted queue at theinitiator and wherein performing the RDMA write further comprises addingthe RDMA request to a posted list at the initiator responsive todetermining that the RDMA write succeeded.
 16. The computer programproduct of claim 10, wherein performing the RDMA write comprises sendingan RDMA request from the initiator to the target and placing the RDMArequest in a posted queue at the initiator and wherein performing theRDMA write further comprises sending a commit message responsive tosuccessful completion of the RDMA write.
 17. An apparatus comprising: atleast one processor; and a memory coupled to the at least one processor,wherein the memory comprises instructions which, when executed by the atleast one processor, cause the at least one processor to implement areplication protocol for replicating memory from an initiator to atarget, wherein the instructions cause the at least one processor to:request, by the initiator, one or more pages of memory at the target;responsive to the initiator receiving a page advertisement from thetarget, update a list of pages at the initiator, wherein the list ofpages is replicated at the target; perform, by the initiator, a remotedirect memory access (RDMA) write to the one or more pages of memory atthe target; responsive to successful completion of the RDMA write,update the list of pages at the initiator, wherein upon completion ofthe RDMA write the list of pages is updated at the target; responsive todisconnecting from the target and subsequently reconnecting to thetarget, receive a login message from the target; send a login replymessage to the target, wherein the login reply message comprises amessage code and a last initiator input/output sequence number (IOSN)that completed without error; and responsive to receiving a login replymessage from the target comprising a last target input/output initiatorsequence number (IOSN) processed by target, remove all messages from aretry queue having an IOSN less than or equal to the last IOSN reportedas processed by target and resume the initiator.